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Subsituting USB power for Battery?

Discussion in 'General Electronics' started by [email protected], Dec 8, 2004.

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  1. h1dd3n_h4m$

    h1dd3n_h4m$ Guest

    I've got a battery powered peripheral (FM Transmitter) that I'd like to power
    from USB.

    I know that USB puts out 5v, and that a "Double A" batter puts out 1.2 v,
    and assume that two would put out the same.

    Can anyone point me in the right direction to make my little project? I
    have a soldering gun, and can use it, but need an idea of what kind of parts
    to put in between.

    Also, is there any danger that dicking around with a USB cable like this
    could cause corruption elsewhere in the USB bus?


    "After we become a strong force as the result of the creation of the state,
    we shall abolish partition and expand to the whole of Palestine. "

    - Isreali founding father David Ben-Gurion

  2. Pham Nuwen

    Pham Nuwen Guest

    Depends if the batteries are in series or parallel. if they are in
    parallel they still put out 1.2v but can produce more current. If they
    are in series then they will put out 2.4v.

    Well to start here is the USB pinout:

    Pin Name Description Cable Colour
    1 Vcc +5V DC Red
    2 D- Data - White
    3 D+ Data + Green
    4 Gnd Ground Black

    Low power, bus-powered function Draw 100 mA (max)

    (your can't actually trust those colors listed, but they might be right)

    Basically you want to use a few resistors so we go to good ol' ohms law..

    ----- = 50 ohms

    so you want to ideally keep this down to 50 ohms total. It just so
    happens 12 ohms aren't the hardest resistors in the world to find.

    12 ohms X 100mA = 1.2v (or 1.25v in reality)
    (3x12 ohms) X 100mA = 3.6v (or 3.75v in reality)

    double check the math.... so here is your circuit (excuse the crude drawing)

    5v + pin 1 ---+[ 12 ohm ]++[ 12 ohm ]++[ 12 ohm ]+---- 1.2v+
    GND - pin 4 ----------+[ 12 ohm ]+--------------+
    +------------------------------- GND -
    While it is possible, I doubt you could do a lot of serious harm, still
    I'd unplug your other usb devices until you tested this and are happy
    with it. Worst you could do is reverse pin 1 and 4 and possible harm the
    "battery" driven device, or short circuit 1 and 4 in which case you
    might cause your computer power supply to trip into short circuit mode.
    Still it isn't like you are playing with high voltage here.

    Lemme know how it goes, this isn't a bad idea, and could be a useful
    little cable to have kicking around.
  3. Randa

    Randa Guest

    You can overload the current capabilities of the USB bus on your
    motherboard and potentialy damage your board. One of the things
    you should determine before you get too excited is what the
    current requirements are for the FM transmitter. Then add in your
    other USB devices and see what your motherboard is willing to

    Now you know why there are active (powered) and passive(USB cable
    powered) USB external hubs.
  4. Bryan Heit

    Bryan Heit Guest

    I have an FM transmitter which can run from either USB or a separate
    battery pack. Batterypack takes 2-AAA batteries. If they can make it
    work you can too, although I imagine some sort of a voltage control
    board may be required.

  5. Your Mom

    Your Mom Guest

    Try using a Voltage REGULATOR, it REGULATES the VOLATGE, mexicans use them
    due to new volatage regulations stipulated by the USA government. Ok now
    read this, as long as you are a white person that can read of course. If
    your mexican you will be expected to know this already.


    THEY HAVE the good volatge regualtors at the action electronicsm, and that
    other MEXICAN guy was right bout the current you gotta make sure you have
    the same RESISTANCE, liek how MEXICANS resist working at WAL MART. So you
    will liekly need a meter, but that is Eroupoean so your proably going to
    spend way to much fucking time wiht this seemingly retarded project unless
    you want'ed to learn more about the mexicans and their power
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